In order to determine the distribution of fluids in oil bearing rock formations from test samples it is common to saturate a core sample of the rock with one of the fluids and to submerge the sample in another fluid contained in a centrifuge chamber. Usually one of the fluids is water and the other is oil from the rock formation. The fluids are thus of different densities and are immiscible. When the centrifuge is rotated some of the fluid in the sample is expelled and is replaced in the vacated pores by the other fluid. By measuring the amount of fluid expelled at equilibrium over a series of incremental speeds the capillary pressure distribution of the sample can be mathematically determined.
Although rock samples have been tested in this manner for many years, the procedure is not without drawbacks. Traditional designs of centrifuges for these procedures are capable of handling only small rock samples. The size of rock samples has been limited by material strengths for high speed centrifuges and by restrictions in mathematical procedures for converting the results of centrifuge experiments into capillary pressure relationships. Rock samples larger than one inch in diameter and one inch in length are seldom tested. Because the rock samples are small, a number of different samples must be tested in order to get test results that can be relied upon to accurately portray conditions in the rock formation. Further, the accuracy of measurement of produced fluid volumes from small rock samples is of concern. In addition, the preferred relatively rapid mathematical differentiation of the raw data produced by the fluid expulsion tests is not always an accurate procedure for reducing raw data from core plug experiments. Without getting into details, accurate reduction of the raw data may require involved mathematical procedures depending on the dimensions of the centrifuge and the rock sample.
It would therefore be desirable to be able to test rock samples in an improved manner, in order to overcome the need to test so many samples and to enable data reduction by differentiation of the raw data.